Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, alcohol and alcohol-based fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. Various engine systems may be used with alcohol fuels, utilizing various engine technologies and injection technologies. Further, various approaches may be used to control such alcohol-fuelled engines to take advantage of the charge-cooling effect of the high octane alcohol fuel, in particular to address engine knocking.
For example, in engines configured with a direct injector for delivering fuel or an alternate knock control fluid to engine cylinders, a pulse width of the direct injection may be adjusted to meet the knock control goals. One example approach is shown by Surnilla et al. in U.S. Pat. No. 8,127,745. Therein, an amount of knock control fluid to be direct injected is determined based on an amount of knock relief required, an amount of knock control fluid that is available, a flow rate of the injector, as well as the charge cooling and octane rating of the knock control fluid being injected. A pulse width of the direct injection is then adjusted based on the determined amount.
However, the inventors herein have identified potential issues with such an approach. As an example, injector settings are adjusted based on values (e.g., flow rate through the injector, charge cooling and octane rating of the knock control fluid, etc.) that are typically determined at nominal conditions. However, the actual conditions at the direct injector may be very different. For example, temperature conditions at the direct injector when the injector is first activated may be very different from conditions when the injector has been activated for a while. Specifically, the temperature of the injector may increase during periods where the direct injector is not performing an injection since the injection of a substance can cool the injector. Thus, when operating at mid engine loads, where a knock control fluid is commonly not used, a cylinder may be fueled using a port injector while an injector tip temperature of a cylinder direct injector may become substantially higher (e.g., around 260° C.). If fuel is injected shortly thereafter (e.g., at higher engine loads) for knock relief, the fuel will be at the elevated temperature. In comparison, during regular engine operation, direct injector temperatures may be substantially lower (e.g., around 100° C.). As such, when at or near room temperature, knock control fluids such as ethanol have a higher heat of vaporization. The heat of vaporization, and therefore the charge cooling potential, then decreases with increasing temperature. Consequently, when use of a knock control fluid is resumed (after a period of not using the injector), a pulse width calculated based on the nominal values may not provide sufficient knock relief due to potentially reduced mass flow and reduced charge cooling.
In one example, some of the above issues may be at least partly addressed by a method for an engine comprising, direct injecting a knock control fluid into an engine cylinder, and adjusting an injection parameter of the direct injection based on a temperature of the knock control fluid at a time of release from a direct injector. The adjusted injection parameter may include a pulse width of the injection, an injection amount, a direct injection system pressure, or a combination thereof. In this way, settings for a knock relieving direct injection may be adjusted based on a real time estimate of the charge cooling potential of the injected knock control fluid.
As an example, in response to knock (or in anticipation of knock), an engine control system may determine an initial injection setting for direct injection of a knock control fluid based on nominal operating conditions. This may include an initial amount of fluid to be injected, a pulse width of the injection, a timing of the injection, etc. The control system may then adjust the initial settings based on an estimated or inferred temperature of the knock control fluid at a time of release from the direct injector. For example, the expected fluid temperature may be estimated or inferred based on an idle period of the injector since a last injection, an amount of knock control fluid injected at the last injection, engine conditions during the idle period, thermal mass of the injector, heat transfer from combustion to the injector, heat transfer from the injector to the coolant, heat transfer from the injector to the knock control fluid, estimated temperature of knock control fluid in a common fuel rail upstream of the injector, etc.
As the duration since a last injection from the direct injector increases, while a port injector continues to inject fuel into an engine cylinder, a temperature of the direct injector may increase. This may cause a temperature of the knock control fluid, at a time of release from the direct injector, to also increase, and a charge cooling effect of the knock control fluid to decrease. Therefore, the control system may adjust the initial settings of the knock control fluid injection with a correction factor based on the estimated increase in temperature (and/or the consequent decrease in charge cooling effect). A correction may also be applied to the expected mass flow rate of the knock control fluid through the hot injector based on an estimated vapor pressure of the knock control fluid at the elevated temperature. Based on the correction, a pulse width of the direct injection of the knock control fluid may be adjusted. For example, as an estimated temperature of the fluid at release increases, a pulse width of the injection may be increased. Additionally, or optionally, an injection quantity of the knock control fluid and/or a direct injection system pressure may be increased. In addition, a predicted deficit in knock relief may be compensated for by adjusting one or more other engine operating parameters. For example, the residual knock relief may be provided via spark timing adjustments, boost adjustments, cam phasing adjustments, EGR adjustments, etc.
In this way, a knock control fluid mass can be adjusted by adjusting injection settings of a knock control fluid in anticipation of changes in knock relieving efficiency due to heating of the knock control fluid at a time of release from an injector. By estimating an expected temperature of the fluid at the time of release based on injector conditions, such as based on whether the injector was already activated or idle, a drop in the charge cooling effect of the knock control fluid can be predicted, and knock relief compensating adjustments may be appropriately made. By increasing the pulse width of direct injection of the knock control fluid at elevated injector temperatures, a knock relieving efficiency of the fluid at release can be improved. In addition, injector fouling and thermal degradation can be reduced. By better addressing engine knock, engine performance can be improved.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.